1. Field of the Invention
The present invention relates to a dampening arrangement for a printing press such as a lithographic press, and particularly to a dampening arrangement having nozzles for jetting dampening solution onto a roller.
2. Description of the Related Art
In lithography, image areas of a substantially smooth printing plate are treated to become lipophilic while non-image areas of the printing plate are treated to become hydrophilic. A dampening solution containing water as its main ingredient and an oil ink are applied to the printing surface of the printing plate, whereby only the image areas retain the oil ink for printing because of the mutual repellence between the dampening solution and the oil ink. Dampening arrangements for supplying dampening solution are divided into the following two types.
According to one type of dampening arrangement, a row of rollers is arranged from a dampening fountain to the printing surface. A rotating roller which is partially immersed in the dampening solution contained in the dampening fountain takes up the dampening solution onto its surface and transfers the dampening solution to a neighboring roller by contact therewith, whereby the dampening solution is transferred to the printing surface and applied thereto.
This type of dampening arrangement allows the dampening solution to be supplied to the surface of the roller in the form of a thin film spread evenly in an axial direction of the roller. However, it is difficult to change a supply rate of the dampening solution on the roller surface in each of the portions extending in the axial direction of the roller. Also, this type has the problem that ink is transferred from the printing surface into the dampening fountain through the row of continuously arranged rollers, leading to contamination of the dampening solution.
According to another type of dampening arrangement, in order to solve the aforesaid problem, a source of dampening solution is separated from the printing surface or from a row of rollers arranged to contact the printing surface. The dampening solution is jetted at the printing surface or at one of the rollers, whereby the supply rate of the dampening solution can be changed in each of portions extending in the axial direction of the roller. This type of arrangement includes a nozzle type dampening arrangement in which the dampening solution is jetted from nozzles, as disclosed in Japanese Patent Application Laid-open (kokai) Nos. 51-59511, 1-110146, and 5-330009.
Japanese Patent Application Laid-open No. 51-59511 discloses a dampening arrangement wherein a dampening solution is supplied to each of plural nozzles by a metering pump at a regulated rate and air is fed by a blower to cause a rapid flow of air so that a mist of the dampening solution is jetted from each of the nozzles. In the dampening arrangement, a drive motor of the measuring pump is controlled so as to run the measuring pump at a desired speed corresponding to the speed of a printing press.
Japanese Patent Application Laid-open No. 1-110146 discloses a dampening arrangement comprising a pump unit for supplying a dampening solution, nozzles for jetting the dampening solution supplied by the pump unit, and a controller to control the jet of the dampening solution from the nozzles in accordance with the printing speed of the printing press.
The timing of jetting the dampening solution is determined, as a number of revolutions of the plate cylinder of the printing press, on the basis of a reference value which is previously set and stored, an adjustment value which is entered in accordance with a printing image corresponding to each nozzle, and a correction value which is previously set and stored for each of printing speeds of the printing press. The nozzles are opened for a predetermined period of time at the thus determined timing, thereby controlling the jet of the dampening solution. That is, the dampening solution is jetted by a predetermined amount each time the plate cylinder of the printing press rotates by the thus obtained number of revolutions.
A specific description is not given, but according to Japanese Patent Application Laid-open No. 1-110146, in addition to the aforesaid control, the jet of the dampening solution may be controlled by changing the time period for jetting the dampening solution, i.e., the time duration for opening nozzles, or the jetting pressure, and also the jet of the dampening solution may be controlled by changing the opening area of a shutter member located ahead of each nozzle.
Japanese Patent Application Laid-open No. 5-330009 discloses a dampening arrangement comprising speed detecting means for detecting the printing speed of a printing press, a memory which stores a supply rate of dampening solution for each combination of printing condition and printing speed of the printing press, jetting means which is connected to a source of dampening solution and an air source by piping and which has a plurality of nozzles to continuously jet a mist of dampening solution, by rapid flow of the air, to a printing plate or the surface of a roller in contact with the printing plate, and pressure control means which is provided in a pipe connecting the source of dampening solution and the jetting means and which controls the pressure of the dampening solution supplied to the jetting means on the basis of the supply rate of the dampening solution stored in the memory.
In the dampening arrangement, the supply rate of the dampening solution is set in accordance with printing conditions such as a printing speed, humidity and temperature, and the thus set supply rate is compared with a pressure in the pipe downstream of the pressure control means to maintain the dampening solution discharged on the downstream side of the pressure control means at a constant pressure, thereby controlling the discharge rate of the dampening solution. In addition, a needle valve is provided on the upstream side of each nozzle to finely adjust the supply rate of the dampening solution, thereby guaranteeing a constant supply rate of the dampening solution.
Usually, liquid jetted from a nozzle does not evenly distribute over an area which the jetted liquid reaches (hereinafter referred to as "distribution area"). In other words, the amount of the liquid decreases at the peripheral portion of the distribution area compared to the rest of the area.
Thus, in the aforesaid conventional nozzle type dampening arrangement, the distance between neighboring nozzles and the distance between the nozzles and a printing plate or a roller at which dampening solution is jetted are set such that the distribution areas of the dampening solution somewhat overlap each other in the widthwise direction of the printing plate or in the axial direction of the roller, whereby neighboring distribution areas compensate each other for thinner application of the dampening solution at a peripheral portion of each distribution area.
Although the reason is unknown, various printing tests using a nozzle type dampening arrangement have revealed that even when the supply rate of dampening solution is adjusted according to printing conditions, a printing defect caused by excess dampening solution occurs on the printing surface at portions corresponding to overlapped portions of neighboring distribution areas of the dampening solution as the printing speed increases.